1. Field of the Invention
The present invention relates to a control technique for performing positioning using a travelling wave driven motor and, more particularly, to a control technique suitable for use in a precision operation mechanism for positioning with a precision on the order of microns or less, such as a semiconductor manufacturing process or an optical system adjustment actuator, and which can achieve high-speed movement within a large operation range, with high precision positioning.
2. Related Background Art
FIG. 2 shows a typical prior art configuration. The arrangement shown in FIG. 2 will be described below. Two phases of frequency powers are supplied to the main body 1 of a known ultrasonic motor (to be abbreviated as USM hereinafter) as outputs from power amplifiers 3. Two phase signals are generated by a known voltage controlled oscillator (to be abbreviated as VCO hereinafter) 5 and a phase shifter 4. The VCO 5 generates a frequency signal having a driving frequency f.sub.d, and the phase shifter 4 selectively outputs two phase signals having a phase difference of 0.degree., and selectively 90.degree. or -90.degree.. The .+-.90.degree. signals correspond to forward and reverse rotations of the USM main body 1. One of the .+-.90.degree. signals is selected by a switch 7, and is input to the USM main body 1. A CPU 6 calculates and outputs a frequency command to be supplied to the VCO 5 on the basis of a detect signal from a speed detector 2 attached to the USM main body 1.
In this manner, the stator vibration frequency f.sub.d is used as a manipulated variable (in this case, a value for manipulating a speed as a control variable), and this system has a basic principle of controlling the motor speed by changing this value.
The static relationship between the motor speed and the driving frequency is represented by an asymmetric curve shown in FIG. 3. In an illustrated region, unimodal characteristics are exhibited, and correspond to resonance characteristics of a stator consisting of an elastic member. A frequency range used in speed control is a right slope, having a relatively small gradient, of the curve, i.e., a region at the high-frequency side of the resonance frequency. However, as will be described below, of these characteristics, information utilized for control is only the fact that the right slope curve is monotonous.
As a typical speed control method by the CPU 6, the following method is known. Based on a speed set value v.sub.r and a detect speed v, the driving frequency f.sub.d is changed according to the following relationship: ##EQU1## where .increment.f is amount the driving frequency is changed per calculation.
The prior art technique depends on speed control. FIG. 2 does not illustrate position control.
Position control is an additional feature, and is achieved by adding a method of stopping a driving signal when a set position is reached to a calculation of the CPU 6. In this system, since a driving operation until the motor is stopped is a unidirectional operation, if the motor overshoots a set position, the forward/reverse rotation switch 7 is switched to operate the motor in the reverse direction.
In the fundamental technique of the above-mentioned prior art, the operation speed is adjusted by operating the driving frequency. For this reason, it is impossible to achieve closed-loop control as well as to continuously switch between forward and reverse rotations. Manipulation of the driving frequency inevitably leads to nonlinear transfer characteristics, and various effective feedback control techniques cannot be utilized. Therefore, the above system suffers from limited performance.
Since the driving frequency is always changed, a problem of unsteadiness cannot be ignored as well as the nonlinear characteristics. A travelling wave driven motor tends to excite various modes which are not associated with the driving force since it utilizes vibration characteristics of an elastic member, and all these modes generate disturbance vibrations. Originally, a coherent vibration system based on a constant frequency is desirable. A major cause of unsteadiness that cannot obtain fixed characteristics is the above-mentioned driving method of the prior art.
For the above-mentioned reasons, it is difficult for the prior art to reliably and precisely adjust the motion of a motor.
It is an object of the present invention to provide a control apparatus for a travelling wave driven motor which can stably operate an ultrasonic motor or a vibration wave driven motor at a low speed.
Other objects of the present invention will become apparent from the following detailed description of the invention.